ox THE AXATOMY OP FISHES. 
249 
transverse process of the fourth vertebra. The anterior wall of each bony recess for 
the air-bladder articulates at its distal end with the post-temporal stem, and fre- 
quently also along its anterior face with the inferior limb of the same bone {e.g., 
Glgptosternum) ; sometimes the inferior limb is extremely slender {e.g., Bagarius); 
exceptionally, it becomes quite rudimentary and loses its usual articulation with the 
basi-occipital {e.g., Clario.s). 
The skull presents no special modifications in any of the fifteen genera above men- 
tioned. Whatever changes the membranous labyrinth of the internal ear may 
undergo, the mode of formation and the relations of the cavum sinus imparis, the 
atria sinus imparis, and the recesses for the sacculi are essentially the same as in the 
Silurldse normales. 
The condition of the air-bladder in this group is singularly varied, and in pro- 
portion to the size of the fish is always small and often diminutive. Many of its 
more characteristic features are clearly the results of atrophy and degeneration. 
The principal modifications appear to be due to the partial or complete suppression of 
the lateral chambers, and the partial or complete subdivision of the anterior chamber 
into two laterally situated cavities or air-sacs, either by the solidification of the 
mesial portion of the bladder, or by more or less complete longitudinal constriction. 
In all cases the atrophied bladder is partially or completely enclosed within bony 
recesses. In one or two instances {e.g., Schilbichthys, and probably also Eutropi- 
ichthys) the air-bladder, although solid mesially, nevertheless retains in each half 
traces of its original and normal division into anterior and lateral compartments, but 
the extreme thickness of its walls, and the small size of its internal cavities, afford 
sufficient proof of its degenerate and functionless condition. Solidification of the 
central portion of the bladder in one instance {e.g., Silondia) has reduced the cavity 
of the organ to the condition of a circular canal surrounding a massive central 
pillar. Two species furnish examples of the extreme variability to which degenerate 
structures are liable. In one {Ailia) the air-bladder assumes the shape of a cylin- 
drical horse- shoe, and is almost solid through the development of internal fibrous 
trabeculae, except at its hollow forwardly-curved cornua ; in the other {Pimelodus 
sapo) the organ is almost solid mesially, or at all events its original cavity is oblite- 
rated by the growth of a dense fibrous netwoi'k, and of its two pairs of forwardly 
directed prolongations one only is hollow and receives the insertion of the tripodes. 
In another instance {Cryptopterus micropus, and possibly also C. hexapterus) the air- 
bladder consists of two partially separated lateral halves, but its degenerate character 
is betrayed by the partial obliteration of the cavities of the greatly reduced lateral 
compartments by a dense network of fibrous bundles. Such facts suggest that the 
partial obliteration of portions of the internal cavity of the bladder, and more 
especially the lateral chambers, by the development therein of anastomosing fibrous 
trabeculae, often precedes their more complete su])pression. But despite such excep- 
tional variations in particular species or genera, the more frequent condition of tlie 
Mucccxciii.— n. 2 K 
